Semiconductor piezoresistive transducers have been widely known for many years and are used in a great variety of applications and harsh environments which require the transducer to be protected from the environment. In addition, in order to provide absolute pressure measurements, the transducer sensing elements must often be contained within a cavity of a known pressure, usually a vacuum, to provide a pressure reference. This vacuum cavity must of course be hermetically sealed to maintain the vacuum and protect the sensing elements. Such transducers are termed "absolute" pressure transducers, and may be fabricated as extremely small devices for use in the medical field, as pressure sensors in automobiles, and so on. The smaller the transducer can be manufactured for the given pressure range, the wider variety of applications that are possible and the cheaper the price.
Techniques for hermetically sealing semiconductor piezoresistive transducers from hostile environments have generally limited how small the transducers could be made since additional lateral space has been required to accommodate a hermetically sealing cover structure. Essentially, the piezoresistive transducer employs silicon resistive elements, the resistance of which varies according to the intensity or magnitude of an applied force upon an associated diaphragm. Such resistors comprise serpentine or tortuous line patterns. It is these resistor elements which must be hermetically isolated from the external environment to ensure proper sensing performance.
An example of such hermetically sealed environmentally protected absolute piezoresistive transducers is disclosed in U.S. Pat. No. 5,002,901 entitled "Method of Making Integral Transducer Structures Employing High Conductivity Surface Features" issued on Mar. 26, 1991 to A. Kurtz et al. and assigned to the assignee herein. The piezoresistive elements in that patent are formed over the central region of a dielectric layer which overlays a silicon diaphragm. The elements are arranged to form a Wheatstone bridge where the four circuit nodes of the bridge are configured as four p+ silicon electrical contact posts disposed on the peripheral comers of the device. Electrical interconnections, also comprised of p+ silicon, interconnect the contact posts with the piezoresistive transducer elements. A bias voltage is brought to two of the contacts, while the voltage is measured between the other two contacts. This is accomplished by bonding external wires to each contact, which wires are run to an external voltage source and potentiometer.
As the silicon diaphragm deflects in response to an applied force or pressure, the resistance of the piezoresistive transducer elements changes, thereby changing the measured voltage. The actual applied force or pressure can then readily be determined from the measured voltage.
The hermetic seal for this device was provided by fabricating a peripheral flange on the device's outer periphery beyond the contact posts. In this way an absolute cavity can be made which will provide a vacuum reference. A glass sheet cover is then bonded to the top of the flange to create the hermetic seal. The glass sheet is also bonded to the outside of the contact posts, while openings are left atop the contact posts to enable subsequent wire bonding thereto. A major drawback of this configuration is that the peripheral flange undesirably increases the size of the overall device. Moreover, the contact posts must be enlarged to provide adequate surface area for bonding to both the glass sheet and to the external wire bonds. This likewise increases the size of the device.
Accordingly, it would be desirable to reduce the size of this type of transducer by eliminating the above described additional space allocated to the hermetically sealing structure of the semiconductor transducer, and the required enlargement of the contact posts.
It is therefore an object of the present invention to provide a reduced size, hermetically sealed semiconductor transducer with a vacuum cavity reference in which the hermetic sealing structure contributes only minimally to the overall size of the device.
It is another object of the present invention to provide an improved method for fabricating such a reduced size, hermetically sealed semiconductor transducer.